Nanoreactors based on hydrophobized tubular aluminosilicates decorated with ruthenium: Highly active and stable catalysts for aromatics hydrogenation

Article abstract of DOI:10.1016/j.cattod.2020.10.001

Industrial hydrogenation catalysts must be not only selective and active but also resistant to feedstock impurities, including water. We report the strategy of preparing catalytic core-shell nanoreactors based on hydrophobized aluminosilicate nanotubes loaded with ruthenium. The modification of halloysite with alkyltriethoxysilanes enhances hydrophobicity of the clay nanotubes (water contact angle up to 122°) and enables their selective loading with 4-nm ruthenium particles. Such a core-shell tubular nanoreactors provide shielding of active sites from deactivation by admixed water and prevent metal leaching. Produced mesoscale catalysts were active in the hydrogenation of aromatics both in organic and aqueous media at 80 °C and a hydrogen pressure of 3 MPa. Benzene hydrogenation in the biphasic system with water resulted in a complete conversion with 100 % selectivity to cyclohexane over halloysite modified by C₁₈-triethoxysilane supported ruthenium catalyst with turnover frequency (TOF) of 4371 h^?1. This catalytic system remained stable after ten cycles of benzene hydrogenation, providing 98 % conversion. The demonstrated synthetic strategy is promising for the design of industrial catalysts for the hydroprocessing water-containing organic feedstock and may be upscaled due to the abundant availability of halloysite clay nanotubes.

Full text of DOI:10.1016/j.cattod.2020.10.001

Catalysis Today xxx (xxxx) xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Nanoreactors based on hydrophobized tubular aluminosilicates decorated
with ruthenium: Highly active and stable catalysts for
aromatics hydrogenation
,
Aleksandr Glotova b,*, Andrei Novikova, Anna Stavitskayaa, Vladimir Nedolivkoa,
Dmitry Kopitsyna, Alexandra Kuchierskayaa, Evgenii Ivanova, Valentine Stytsenkoa,
,
Vladimir Vinokurova, Yuri Lvovc **
a Gubkin Russian State University of Oil and Gas, Department of Chemical Technology and Ecology, Division of Physical and Colloid Chemistry, 65 Leninsky Prosp.,
119991 Moscow, Russian Federation
b Lomonosov Moscow State University, Department of Chemistry, Division of Petroleum Chemistry and Organic Catalysis, 1 Leninskie Gory, 119991 Moscow, Russian
Federation
c Louisiana Tech University, Institute for Micromanufacturing, 505 Tech Drive, Ruston, LA, 71272, USA
A R T I C L E I N F O
A B S T R A C T
Keywords:
Industrial hydrogenation catalysts must be not only selective and active but also resistant to feedstock impurities,
including water. We report the strategy of preparing catalytic core-shell nanoreactors based on hydrophobized
aluminosilicate nanotubes loaded with ruthenium. The modification of halloysite with alkyltriethoxysilanes
enhances hydrophobicity of the clay nanotubes (water contact angle up to 122◦) and enables their selective
loading with 4-nm ruthenium particles. Such a core-shell tubular nanoreactors provide shielding of active sites
from deactivation by admixed water and prevent metal leaching. Produced mesoscale catalysts were active in the
hydrogenation of aromatics both in organic and aqueous media at 80 ◦C and a hydrogen pressure of 3 MPa.
Benzene hydrogenation in the biphasic system with water resulted in a complete conversion with 100 %
selectivity to cyclohexane over halloysite modified by C18-triethoxysilane supported ruthenium catalyst with
turnover frequency (TOF) of 4371 hꢀ 1. This catalytic system remained stable after ten cycles of benzene hy-
drogenation, providing 98 % conversion. The demonstrated synthetic strategy is promising for the design of
industrial catalysts for the hydroprocessing water-containing organic feedstock and may be upscaled due to the
abundant availability of halloysite clay nanotubes.
Nanoreactors
Halloysite nanotubes
Ruthenium nanoparticles
Hydrophobicity
Silylation
Aromatics
1. Introduction
including zeolites [10–13]. These supports are characterized by high
specific surface area, narrow particle size distribution, and good thermal
Conventional industrial heterogeneous catalysts mostly comprise a
support and active phase immobilized on the surface or within the pore
system and represented by micro or nanoparticles [1–5]. These active
sites assure chemicals’ transformation with enhanced reaction rates.
Catalyst’s support is often multipurpose and could be functional (acid or
base-catalyzed reactions, feedstock molecules pretreating) or
non-functional (provides resistance to attrition, shape-selectivity, active
particles formation, stabilization, and protection) [5–9]. The commonly
used supports are alumina, silica, titania, and aluminosilicates,
and chemical stability in the presence of water and various pH condi-
tions [14–17].
Despite numerous studies of catalysts supports, the search for
promising new materials combining advantages of synthetic alumino-
silicates with low cost and availability of natural clays is relevant
[18–21]. Among natural aluminosilicates, halloysite clay nanotubes are
the most promising supports for functional materials and catalysts with
advanced properties [7,22–25]. The unique features of halloysite are
opened tubular structure with 50ꢀ 70 nm diameter, 0.5–1.0 μm length,
* Corresponding author at: Gubkin Russian State University of Oil and Gas, Department of Chemical Technology and Ecology, Division of Physical and Colloid
Chemistry, 65 Leninsky prosp., 119991 Moscow, Russian Federation.
** Corresponding author at: Louisiana Tech University, Institute for Micromanufacturing, 505 Tech Drive, Ruston, LA, 71272, USA.
E-mail addresses: glotov.a@gubkin.ru (A. Glotov), ylvov@latech.edu (Y. Lvov).
https://doi.org/10.1016/j.cattod.2020.10.001
Received 14 August 2020; Received in revised form 19 September 2020; Accepted 5 October 2020
Available online 12 October 2020
0920-5861/© 2020 Elsevier B.V. All rights reserved.
Please cite this article as: Aleksandr Glotov, Catalysis Today, https://doi.org/10.1016/j.cattod.2020.10.001

A. Glotov et al.
Catalysis Today xxx (xxxx) xxx
and oppositely charged outer and inner surface of different chemical
composition (SiO2 and Al2O3). These features open the way for nano-
architectural constructions with optimized system properties [26–28].
The most interesting is the selective loading of metal nanoparticles
either onto the outer surface or inside the lumen of these clay tubes
[29–31]. This approach is promising for the design of catalysts resistant
to the deactivation by the impurities in the feedstock. Such core-shell
catalysts are nanoreactors, ensuring the selective transport of reactant
molecules to the active sites encapsulated in the tubes. The chemical
reaction catalyzed by such encased metal sites and the efficient removal
of desired products from the reactor cavity were demonstrated [32,33].
The idea of shielded core-shell catalysts, where active sites are hid-
den inside the reactors, is actively pursued; for example, dendritic
polymers and micelles were employed as “nanoreactors” [34,35].
However, these formulations’ recycling stability and scalability were not
achieved. The mesoporous silica also may be considered as nano-
reactors, and it is closer to the industrial applications [36,37]. Porous
silica particles with palladium active sites were modified with per-
fluorodecyltriethoxysilane to ensure their hydrophobicity and transport
of hydrogen via the gas phase in the gas-liquid-solid triphase process
[38]. However, large-scale production of such modified silica is unfea-
sible due to the prohibitively high cost of perfluoroalkylsilanes. The
mesoporous silica or aluminosilicates of any type (MCM-41, SBA-15, 16,
HMS) are formed by multiple channels, where the composition of each
pore after modification and deposition of metals cannot be ensured and
properly characterized [6,8,16,39].
2. Experimental
Following reactants were used: halloysite (HNT) (Sigma-Aldrich),
propyltriethoxysilane (PTES), octyltriethoxysilane (OTES), and octade-
cyltriethoxysilane (ODTES) (Sigma-Aldrich), ruthenium (III) chloride
(Aldrich, 45–55 % Ru), sodium borohydride (85 %, Sigma-Aldrich).
Benzene, toluene, ethylbenzene, hydrogen peroxide (37 %), 2-propanol,
each chemically pure, were purchased from Ekos-1 (Moscow, Russia).
Pure gases (Ar, H2, He, N2) were supplied by NIIKM (Moscow, Russia)
and additionally purified by passing them through cold traps filled with
CaA zeolite.
2.1. Halloysite (HNT) silanization
Halloysite was pretreated by boiling with hydrogen peroxide for 2 h
to remove all organic impurities. After cooling and decantation, HNT
◦
was washed and dried at 100 C. The halloysite surface was modified
with propyl-, octyl-, and octadecyltriethoxysilane to prepare catalysts
with different hydrophobicity. For this purpose, silanes (0.5 mL) were
added dropwise to toluene dispersion of halloysite (3 g in 20 mL) and
stirred for 18 h. The modified HNT was centrifuged, washed with
toluene, and dried at 60 ◦C for 12 h. The resulted materials were named
correspondingly HNT-C3, HNT-C8, HNT-C18, indicating elongation of
alkyl chains (3, 8, and 18) attached to the tubes outermost surface.
2.2. Synthesis of HNT/Ru core-shell catalysts
With the appropriate nano/micro-scale roughness, the alkyl-
modified surfaces provide the contact angles sufficient for the super-
hydrophobic effect, combining efficiency and feasibility [40]. The in-
dustrial applicability of this approach was recently shown in
superhydrophobic wire mesh demister employed in the gas-liquid
separator [40], antimicrobial superhydrophobic cotton fabrics [41],
and aluminum-alloy surfaces [42]. The required surface roughness is
readily obtainable using the low-cost natural clay nanotubes such as
palygorskite [43] and halloysite [44]. Aluminosilicate halloysite clay is
a promising material for nanoreactor synthesis due to its unique tubular
structure formed by kaolinite layers rolling into tiny tubes [3,7,20,
45–47].
A weighed ruthenium (III) chloride (60 mg) was dissolved in 40 ml of
distilled water. This solution was added to 1 g of the support (pristine
HNT, and modified HNT-C3, HNT-C8, HNT-C18); resulting mixture was
sonicated for 10 min and treated with microwave radiation at 800 W for
3 min. This material was decanted and washed with distilled water. The
reduction of Ru3+ to Ru◦ was performed using 0.5 M aqueous solution of
sodium borohydride (excess). After reduction, the Ru-nanoclay catalyst
was centrifuged, washed with distilled water to remove the products of
sodium borohydride decomposition and air-dried for 24 h at 50 ◦C. The
resulted metal-clay materials were named Ru@HNT-0, Ru@HNT-C3,
Ru@HNT-C8, Ru@HNT-C18
.
Hydrogenation of aromatics is the essential process of petroleum
chemistry. For example, benzene is hydrogenated to cyclohexene and
cyclohexane, which are further converted into caprolactam, cyclo-
hexanol, and adipic acid. The same route for toluene provides methyl-
cyclohexane that is oxidized to methyladipic and glutaric acids [48–50].
On the other hand, in refineries, motor fuel hydroprocessing, including
dearomatization, is a large-scale process, with permanently increasing
applications [51–53]. The benzene hydrogenation is the most common
model reaction to test the supported metal catalysts [54,55].
Ruthenium-containing catalysts are widely studied for the hydrogena-
tion of aromatics to cycloalkanes in the liquid phase [55]. Despite these
catalysts’ high activity, they are sensitive to the presence of water in a
feedstock. Many works are devoted to the partial benzene hydrogena-
tion to cyclohexene caused by water or organic/inorganic additives [55,
56]. The reasons for decreasing benzene conversion to cyclohexane via
stopping the reaction by water addition were discussed as competitive
adsorption on Ru-active sites, different solubility, and mass transfer of
reactants, hydrogen, and products within organic and aqueous phases
[55,57–59].
2.3. Physicochemical characterization
The structure and morphology of the catalysts were studied by
transmission electron microscopy (TEM) using a JEM-2100 setup (Jeol,
Tokyo, Japan). The samples were placed on the copper grid and
analyzed under an accelerating voltage of 200 kV. The micrographs
were processed using Image-Pro Plus 6.0 software. Particle size distri-
bution was analyzed using 5 microphotographs based on the counting of
500 particles.
Hydrogen temperature-programmed reduction (H2-TPR) was carried
out on an AutoChem HP2950 (Micromeritics, USA). Before experiments,
◦
the catalyst samples were calcined in air at 400 C for 4 h to transfer
ruthenium into the oxide form. The test sample with a weight of ~0.1 g
was placed in a quartz reactor and kept in an argon flow at 400 ◦C for 1
h. Then the reactor was cooled down to 60 ◦C, and the sample is heated
in the 8 % H2+Ar gas mixture (30 mL/min) at 10 ◦C/min up to 400 ◦C.
Specific surface area (SBET), volume (Vp), and diameter (Dp) of pores
were determined by low-temperature nitrogen adsorption/desorption
using a Gemini VII 2390 t (Micromeritics, USA) instrument at 77 K.
Before the measurements, the samples were treated in vacuum at
80–300 ◦C for 4 h. The specific surface was calculated according to the
Brunauer–Emmett–Teller (BET) method in a relative pressure range of
0.04 to 0.25 from the desorption data. The volume of pores and their
diameter were estimated in terms of the Barrett–Joyner–Halenda model.
The elemental composition of the catalyst formulations was deter-
mined using an ARL Perform’X X-ray fluorescence spectrometer
(Thermo Fischer, USA). Processing and refinement of the results were
In this work, we present an architectural approach for metal core -
ceramic shell nanoreactors based on natural aluminosilicate nanotubes
loaded with ruthenium. The key feature for the catalyst’s high stability
and efficiency is the modification of the nanotubes with alkyltriethox-
ysilanes providing hydrophobic outermost surface and selective loading
of noble metal particles in the tubes’ lumens. The synthetic method
proposed ensures the formation of the core-shell tubular mesocatalyst
with high activity in the aromatics hydrogenation, water tolerance, and
good recyclability.
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A. Glotov et al.
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Fig. 1. AFM (a), SEM (b), and TEM (c) micrographs of the pristine nanoclay, and the scheme (d) of the hydrophobization and ruthenium deposition.
performed with the UniQuant method. The three-phase contact angle of
the samples was measured with the DSA100 apparatus (Kruss, Ger-
many). The sample was pressed into a tablet (at ca. 100 bar) and placed
into the water-jacketed cuvette filled with cyclohexane. The sample was
autoclave, sealed, filled with hydrogen and the tests were run at pressure
of 3.0 MPa and 80 ◦C. Hydrogenation products were analyzed on a
Chromos GC-1000 gas chromatograph equipped with a flame ionization
detector and a MEGA-Wax Spirit column.
thermostated at 20 ◦C. Then a water droplet (1.0–1.2
μL) was added onto
The catalyst’ activity in hydrogenation (turnover frequency, TOF)
was calculated as the amount of reacted aromatic substrate in definite
time (conversion of 40–80 %) per mole of ruthenium specific surface
area, according to the formula: TOF = 2000×Csubs/ti, where 2000 – a
molar ratio of substrate/Ru, Csubs – substrate conversion, ti is the time,
for which substrate conversion (Csubs) was evaluated. Each experiment
was carried out three times at the same conditions, with the results
differing by no more than 2 % from the corresponding average value.
The measurement error did not exceed 1 %.
a sample tablet surface, and the contact angle was measured for 30 s.
2.4. Catalytic experiments
Evaluation of the catalytic activity in hydrogenation of aromatics
was performed with a 5000 Multiple Reactor System (Parr, USA) with
stainless steel batch reactors, Teflon inlet, and magnetic stirrer. The
reactor was loaded with 60 mg of the catalyst and aromatic substrate
(benzene, toluene or ethylbenzene) to keep a constant substrate/Ru
molar ratio of 2000. In case of biphasic hydrogenation tests, the same
quantity of aromatics and distilled water were added. Catalytic tests
were run at a hydrogen pressure of 3.0 MPa and 80 ◦C.
3. Results and discussion
3.1. Optimization of the core-shell tubule system build-up
After reaction, the reactor was cooled down, the pressure was
dropped to atmospheric, and the catalyst was separated from reaction
products via centrifugation, washed 3 times with ethanol, and dried at
50 ◦C overnight. Recycling experiments were performed as for fresh
catalyst: a sample of spent catalysts (60 mg) with aromatic substrate
(benzene, toluene, ethylbenzene or mixture with water) were placed in
The AFM, SEM, and TEM micrographs of pristine halloysite (HNT)
and the scheme of ruthenium-containing catalysts preparation are pre-
sented in Fig. 1. This nanoarchitectural approach comprises the design
of the outer hydrophobic shell, which prevents adsorption of Ru-cations
onto negative nanotubes outermost surface and forces them into the
Fig. 2. Time-dependent contact angles (a) and Sessile drop micrographs for Ru@HNT-C18 (b), Ru@HNT-C8 (c), and Ru@HNT-C3 (d). The solid lines are log-
polynomial fits for triplicate measurements, shades are 95 % prediction intervals, points are measured data series closest to the fit. Scale bars are 0.5 mm.
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Fig. 3. TEM micrographs (a,c,e,g) and Ru-particles size distributions (b,d,f,h) of Ru@HNT-0 (a,b), Ru@HNT-C3 (c,d), Ru@HNT-C8 (e,f), Ru@HNT-C18 (g,h) catalysts.
hydrophilic lumens. We optimized the hydrophobic shell formation
using alkyltriethoxysilanes with the tail length of C3, C8, and C18
samples following the same procedure described in [5,7,32,45,49].
The hydrophobicity of resulting materials increases with the length
of the alkyl attached to the halloysite surface, as indicated by higher
water–cyclohexane–solid contact angle, Fig. 2 [60]. The Ru@HNT-0
contact angle was estimated as rather low (< 10◦); it was difficult to
get a precise value due to the water droplet’s rapid imbibition by the
HNT tablet. The silanized samples showed increasingly high contact
angles, from 50 to 122◦. The silanization prevents adsorption of the
.
Therefore, these samples are referred to as: #1 unmodified nanoclay
(reference catalyst Ru@HNT-0), and three formulations used hydro-
phobized tubes surface, as follows, #2 modified with propyltriethox-
ysilane, PTES (Ru@HNT-C3), #3 modified with octyltriethoxysilane,
OTES (Ru@HNT-C8), and #4 with octadecyltriethoxysilane, ODTES
(Ru@HNT-C18). Microwave-assisted Ru deposition was applied for all
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Table 1
The composition and structural characteristics of pristine halloysite, Ru@HNT-0, and Ru@HNT-C3-18 samples.
Textural characteristics
Metal content,
Ru average particle
Sample
ATES1 alkyl
Ru location2
n/a
SBET, m2
Average pore volume, cm3/g, ±0.1
Average pore
wt.%, ±0.1
size, nm, ±0.1
/g, ± 1
diameter, nm, ±0.1
Halloysite
–
64
58
53
49
47
0.33
0.30
0.28
0.26
0.25
6.4
6.2
6.1
5.9
5.8
none
–
Ru@HNT-0
Ru@HNT-C3
Ru@HNT-C8
Ru@HNT-C18
1.8
2.5
2.7
2.7
none
mostly outer sites
mixed inner / outer
inner sites
1.3
3.4
3.7
4.8
propyl
octyl
octadecyl
inner sites
1
Alkyl-triethoxysilane modifying agent.
Based on TEM data.
2
water molecules by a modified HNT sample, and the longer the alkyl
moiety of the used silane, the higher is the contact angle, as it was also
indicated for other silane coatings on silica [61,62]. These data confirm
the comparable enhancement of grafting density of alkyl chains onto the
halloysite nanotubes: Ru@HNT-C18>Ru@HNT-C8>Ru@HNT-C3>
Ru@HNT-0 (non-modified).
particle size to 4.8 nm was noticed for the Ru@HNT-C18 sample modi-
fied with the silane with the longest alkyl tail (Fig. 3, g, h). It could be
explained by the higher hydrophobicity of octadecyl-modified halloysite
hindering the contact with the ruthenium precursor solution, and thus
fewer nucleating sites available for the formation of the nanoparticles.
The location, dispersion, and availability of active sites impact on the
activity and stability of hydrogenation catalysts. The proposed synthesis
approach opens the possibility of manipulating hydrophobicity and, as a
result, controlling the location of the metal particle deposition. An in-
crease in Ru content was achieved by clay nanotubes’ surface modifi-
cation with silanes, as shown in Table 1.
Due to the increased hydrophobicity of the modified Ru@HNT-C3ꢀ 18
catalysts, their catalytic activity should be different in the presence of
admixed water. We may expect that the samples Ru@HNT-C8 and
Ru@HNT-C18 will prevent water contact with the active sites of the
catalysts due to their contact angle being higher than 90◦, determining
their preferential wettability with the organic phase [60].
The Ru-catalysts based on halloysite hydrophobized with alkylsi-
lanes (propyl-, octyl-, and octadecyltriethoxysilane) grafted to the
nanoclay surface have similar metal content, surface area, pore volume,
and diameter. The modification with silanes provides almost quantita-
tive loading of ruthenium (≈91 % from theoretical), while for the
Ru@HNT-0 sample synthesized on unmodified clay nanotubes, this
parameter does not exceed 60 %. Nitrogen low-temperature adsorption
shows that the volume and diameter of pores monotonically decrease
with an elongation of the grafted alkyl chains of silanes, Table 1. The
silanization changes the route of the Ru nanoparticles formation on the
halloysite surface, Fig. 3. The metal cations are efficiently pushed inside
tubes resulting in the selective formation of Ru◦ encased in the clay
nanotubes.
Modification of the nanotubes may alter ruthenium nucleation on the
HNT surface. Fig. 3a presents the TEM image of Ru@HNT-0 obtained
using the microwave-assisted deposition of ruthenium ions on unmod-
ified HNT followed by reduction. Such a synthesis allows for ultrafine Ru
nanoparticles with a size of ~1.3 nm seeded mostly on the outer HNT
surface, because the inner lumen is charged positively, and ruthenium
cations intercalation to the inner cavity is hindered [32,63]. Under the
given electrostatic conditions, Ru3+ mainly interacts with the negatively
charged outer surface of the unmodified HNT-0 sample.
Fig. 3c–g show the morphology of hydrophobized Ru@HNT-C3,
Ru@HNT-C8, and Ru@HNT-C18. In contrast to Ru@HNT-0, these sam-
ples have Ru-particles located mainly inside the lumen, and their
average diameter increased to 3ꢀ 5 nm. The preferential binding of the
alkylsilanes onto the outer silica surface of the nanotubes leaves only the
inner alumina surface available for the loading of ruthenium. Thus, we
can expect the formation of larger Ru particles on silanized supports due
to the less available halloysite surface for the nucleation and hindered
mass transport of the metal cations from aqueous media [47,64]. The
particle size distributions of Ru@HNT-C3, Ru@HNT-C8 samples are
quite similar, with an average of 3.4–3.7 nm. An increase in the average
The temperature-programmed reduction experiments also confirm
the differences in the ruthenium particle formation. The TPR-H2 profiles
obtained for HNT/Ru samples and their quantification (based on
deconvolution) are presented in Fig. 4 and Table 2. The ruthenium
content was calculated based on hydrogen consumption caused by a
complete reduction of RuO2. All the TPR-H2 profiles of ruthenium cat-
alysts have an intense peak at 123 ± 2 ◦C corresponded to RuO2
adsorbed on the outer surface of the nanotubes [32]. For the Ru@HNT-0
Fig. 4. TPR-H2 profiles for the samples Ru@HNT-0 to Ru@HNT-C3-18 and their deconvolution.
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Table 2
Data of quantification TPR-H2 spectra for Ru@HNT-0 to Ru@HNT-C3-18 samples. Ruthenium content is based on TPR-H2 data.
Ruthenium content, wt.%, ±0.01
Sample
Temperature of reduction, ◦
C
Hydrogen consumption for RuOx reduction, mmol/g, ±0.01
Per the peak of reduction
Total
Ru@HNT-0
Ru@HNT-C3
Ru@HNT-C8
Ru@HNT-C18
125
0.25
0.13
1.40
2.45
2.62
2.66
123/146/>200
120/144/>200
124/157/>200
0.13/0.34/0.02
0.09/0.39/0.04
0.09/0.34/0.10
0.65/1.7/0.1
0.45/1.97/0.2
0.45/1.71/0.5
Fig. 5. Aromatics hydrogenation in organic (a,c,e) and biphasic water-containing (b,d,f) systems over Ru@HNT-0 and Ru@HNT-C3-18 catalysts.
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Table 3
Table 4
Turnover frequencies (TOFs) for aromatics hydrogenation over Ru@HNT-0 to
Ru@HNT-C18 catalysts.
Ru content and textural properties of Ru@HNT-C18 catalyst after recycling.
Textural properties
Recycle
#
Ru content, wt.%,
Organic phase
Biphasic
SBET, m2/g,
Vpore, cm3/g, ±
Dpore, nm,
Substrate
Catalysts
± 0.1
TOF, hꢀ 1, ±10
TOF, hꢀ 1, ±10
± 1
0.05
± 1
Ru@HNT-0
Ru@HNT-C3
Ru@HNT-C8
Ru@HNT-C18
Ru@HNT-0
Ru@HNT-C3
Ru@HNT-C8
Ru@HNT-C18
Ru@HNT-0
Ru@HNT-C3
Ru@HNT-C8
Ru@HNT-C18
4200
3840
4871
8090
6582
3240
3178
2760
1543
1663
2161
2666
2136
2379
2833
4371
1660
2308
3233
2583
776
0
2.7
2.6
2.6
2.6
2.5
2.4
47
47
49
50
50
51
0.25
0.26
0.26
0.26
0.26
0.27
5.8
5.9
6.0
6.0
6.0
6.0
1
Benzene
2
3
5
10
Toluene
The silanized catalysts provided a high activity, expressed in high
TOFs (Table 3), and complete aromatic substrates conversions (Fig. 5).
In all experiments, a selectivity to complete hydrogenation products
(cyclohexane, methylcyclohexane, ethylcyclohexane) was 100 %. As to
the non-modified ruthenium sample Ru@HNT-0, benzene and toluene
conversions stopped at ~80 %, and for ethylbenzene did not exceed 70
% typical for other supported ruthenium catalysts [14,49,55,59].
Despite the similar ruthenium content for all Ru@HNT-C3 to
Ru@HNT-C18 samples, the higher catalytic activity was observed for
Ru@HNT-C18 due to the highly dispersed nanoparticles inside the tubes
(Tables 1,2). It should be noted that the conversion curve for ethyl-
benzene over the Ru@HNT-C3 sample is middling between the
Ru@HNT-C18 and the Ru@HNT-0. This could be explained by the
location of the nanoparticles: for Ru@HNT-0 and Ru@HNT-C3 both
outside/inside and for Ru@HNT-C8 and Ru@HNT-C18 – inside the tubes
only (Fig. 3). This difference is distinctly manifested in biphasic
organic-water systems during benzene, toluene, and ethylbenzene hy-
drogenation. The estimated ratio of activity Ru@HNT-C18 to Ru@HNT-0
is about two, and the final conversion difference is 30–60 %. The sam-
ples hydrophobized with C3-C8 alkyl tails have intermediate
characteristics.
1287
1256
1218
Ethylbenzene
sample, hydrogen consumption occurs only at this temperature, indi-
cating that all the ruthenium nanoparticles deposited on the outer sur-
face. This observation is in full agreement with TEM images, Fig. 3.
Calculated Ru content is lower than measured by XRF (1.80 vs. 1.35 wt.
%) corresponding to the available active sites for interaction with
hydrogen [65]. Rising the silane alkyl chain length in the course of Ru
site-dependent synthesis, results in increase of the number of the
nanoparticles inside the lumen (Table 1, Fig. 3).
As Table 2 shows, for the Ru@HNT-C8 and Ru@HNT-C18 samples,
the first peak area decreases with a simultaneously rise in hydrogen
consumption at 144ꢀ 157 ◦C that could be ascribed to the reduction of
small (diameter ≤2 nm) Ru-particles located in the lumen. For the
Ru@HNT-C18 sample with the longest alkyl chain, one may observe the
much higher hydrogen consumption in the high-temperature range
(above 200 ◦C). Based on the structural characteristics, we may explain
this part of the profiles as a reduction of ruthenium oxides strongly
bonded with the inner alumina layer in the clay lumen [45,49]. For the
Ru@HNT-C18 sample, it may also be caused by particle enlargement.
According to TPR-H2 data, ruthenium content in Ru@HNT-0 to
Ru@HNT-C18 samples is 1.40–2.65 wt. %; the longer the alkyl in the
silane, the more ruthenium oxide was reduced at higher temperatures.
Hydrogen consumption (RuOx reduction) at temperatures >200 ◦C
monotonically increases from 0.02 to 0.04 and 0.10 mmol/g for the
samples hydrophobized with C3-, C8-, and C18-alkylsilanes. These data
are in agreement with the results of elemental analysis (Table 1) and
confirm that the modification of the halloysite surface with ATES pro-
motes the deposition of Ru nanoparticles inside nanotubes’ lumen.
Based on structural characteristics, TPR-H2, and TEM analysis, we
confirm the selective loading of Ru inside clay tubes modified with
alkyltriethoxysilanes. The halloysite hydrophobization achieved by the
alkyl grafting provides a selective deposition of ruthenium cations inside
the nanotubes. The proposed method results in ruthenium nanoparticle
core shielded by clay nanotubes allowing for effective reactors for aro-
matics hydrogenation even with water impurity. Encasing catalytic
metal nanoparticles in the ceramic shells prevents their aggregation at
high temperature and catalysts maintain the high activity at 400ꢀ 500
◦C, as was demonstrated for halloysite-based automotive exhaust cata-
lysts [23,33,66].
For the catalyst Ru@HNT-0 with the outer location of metal nano-
particles, benzene hydrogenation is incomplete even in pure hydrocar-
bon media, and reaction being stopped at 70–80 % conversion. This
“stop effect” is pronounced in biphasic systems where final aromatics
conversion was not higher than 40 %. Therefore, water negatively af-
fects hydrogenation on the external surface of HNT/Ru. In contrast, the
“stop effect” is eliminated for the catalysts Ru@HNT-C8 and Ru@HNT-
C
18, where Ru-particles are located inside the nanotubes. As to the “stop-
effect” observed, it may be induced by concurrent adsorption of water
on Ru-nanoparticles [7,31,48,55]. In all the biphasic systems, we
observed that water addition decreases both the initial activity and the
conversion. Based on TEM (Fig. 3) and catalyst testing data in organic
and organic-water systems, we conclude that the activity and water
tolerance of the nanoclay catalysts are much improved for the samples
where Ru is located inside the halloysite nanotubes.
Notably, the hydrophobized ruthenium containing nanosystems
based on halloysite clay provided superior catalytic activity compared to
montmorillonite, hydrotalcite, and carbon nanotubes supported ruthe-
nium counterparts (TOFs are 270, 1300, and 649 hꢀ 1 at 100, 120, and 70
◦C respectively, with 100 % selectivity to cyclohexane at complete
benzene conversion) in organic media [67–69]. In aqueous media, these
catalysts lose activity due to strong adsorption of water molecules over
ruthenium active sites. As well, cyclohexene’ diffusion gradient prevents
further hydrogenation, which resulted in benzene conversion and
selectivity loss [45,55]. This catalyst worsening was not observed for
hydrophobized shielded core-shell nanoreactors, which only partially
decreased activity upon shifting to water containing media, but retained
the complete conversion of the aromatic substrate at 100 % selectivity to
the product of complete hydrogenation (Table 4 and Fig. 5).
3.2. Catalytic testing
The catalytic properties of Ru@HNT samples were compared in the
hydrogenation of aromatics both in hydrocarbon and in biphasic
hydrocarbon-water media (volume ratio 1:1). We evaluated the initial
hydrogenation activity (as a slope of “conversion-time” curves), the final
aromatics conversions at the end of the test (200 min), and the influence
of added water on the characteristics above. The results for the hydro-
genation of benzene, toluene, and ethylbenzene are summarized in
Fig. 5 and Table 3.
Finally, we have investigated the stability of the most efficient
Ru@HNT-C18 catalyst in the recycling mode of benzene hydrogenation.
As shown in Fig. 6, the catalytic activity expressed in TOF only slightly
7

A. Glotov et al.
Catalysis Today xxx (xxxx) xxx
Fig. 6. Kinetics curves (a) and the final conversions (b) for recycle tests of Ru@HNT-C18 catalyst in a biphasic system benzene-water (50 vol. %).
decreases with the cycle number. This could be explained in partial
metal leaching that is confirmed by Ru content decreases from 2.7 to 2.4
wt.% (Table 4).
Methodology. Yuri Lvov: Conceptualization, Supervision, Project
administration, Writing - review & editing.
The differences between conversion-time curves for 1–3 cycles are
negligible, and conversion for 3 h remains unchanged (100 %). With the
number of cycles, the conversion rate decreased by 2 % and equaled to
98 % in the tenth cycle. Notably, after ten cycles, ruthenium content was
2.4 %, making Ru@HNT-C18 highly active and stable nanoreactors for
aromatics hydrogenation both in organics and biphasic with water
systems.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
4. Conclusions
This work was funded by the Ministry of Science and Higher Edu-
cation of the Russian Federation (Grant No. 14.Z50.31.0035). The au-
thors thank A. Vutolkina, Ya. Chudakov, M. Artemova, E. Smirnova, and
E. Novikova (all are from Gubkin University) for their input and R.
Fakhrullin for AFM imaging of halloysite.
The mesoporous hydrophobized ruthenium-containing catalysts
templated on natural clay nanotubes were synthesized and tested in
aromatics hydrogenation both in organic and biphasic media containing
50 vol % water. By measuring the contact angle, it was established that
the longer is the alkyl moiety of the used silane, the greater is the water
contact angle: from 50 to 122◦ for alkyl chains from 3 to 18. The hy-
drophobic shell allowed the loading of Ru-particles selectively inside the
nanotubes, producing active, stable, and recyclable mesocatalyst. The
longest silane alkyl chains shell (C18) provided practically complete Ru
loading (2.7 wt. % Ru vs. 3.0 theoretical value) for the modified hal-
loysite nanotubes. As a result, 4.6–5.0 nm in diameter metal particles
were synthesized selectively in the lumen of the tubes. This was
confirmed by TEM images and TPR-H2 measurements, resulting in the
peak of hydrogen consumption shifting to a high-temperature area.
The proposed nano-architectural approach resulted in the ruthenium
particle core shielded by clay nanotubes with high catalytic activity in
hydrogenation of aromatics. This core-shell formulation remarkably
enhanced the catalyst water tolerance and stability. Octadecyltriethox-
ysilane modified clay nanotubes support for ruthenium catalyst
(Ru@HNT-C18) was the most active in benzene hydrogenation both in
organic (TOF =8090 hꢀ 1) and biphasic system with water (TOF =4371
hꢀ 1), demonstrating a complete conversion and selectivity to cyclo-
hexane of 100 %. This system allowed for ten cycles of catalytic hy-
drogenation with complete conversion of aromatics and could be scaled
up for industrial applications due to the low cost and availability of
natural clay nanotubes.
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